1. Field of the Invention
The present invention relates to a lighting apparatus, a control method therefor and a backlight apparatus.
2. Description of the Related Art
A liquid crystal display device is a display device making use of optical transparency of a liquid crystal panel, and serves to carry out image display by controlling transmission or shielding (blocking) of the light, through a liquid crystal panel, emitted from a backlight arranged on a back face of the liquid crystal panel.
A plurality of light emitting diodes (hereinafter, LEDs: Light Emitting Diodes) are used as light sources of a backlight. As LEDs used for light sources, there are, for example, white LEDs, color LEDs composed of three primary colors of red LEDs, green LEDs and blue LEDs, etc.
Adjustment methods for the brightness (luminance) and color of LEDs include current value control, PWM (Pulse Width Modulation) control, and so on. In PWM control, adjustment of brightness or white balance is carried out by adjusting the duty ratio of a turn-on period of time and a turn-off period of time of each LED.
The brightness (luminance) of LEDs changes according to the temperature characteristics of their devices, or the aged deterioration of long-term use, and besides their individual differences. Accordingly, there is a technique in which, in order to maintain the brightness or brightness of a backlight constant, feedback control is carried out on the backlight by using various kinds of sensors such as brightness sensors arranged in a backlight housing.
In addition, there is also a technique in which a backlight is divided into a plurality of blocks, and light emissions of light sources are controlled independently of one another for each of the blocks. According to this technique, it becomes possible to perform, for example, local dimming control in which the brightnesses of light sources are made to differ for each block according to an image signal to be inputted, in such a manner that the brightness of a light source for a block corresponding to a low gradation (gray level) portion is made low, whereas the brightness of a light source for a block corresponding to a high gradation portion is made high. In such a backlight, it is desirable to obtain a measured value of brightness for each block by means of a sensor, and to carry out feedback control for the stabilization of brightness based on the measured brightness value thus obtained.
In the feedback control for stabilizing the brightness of a backlight, the following method is known as a method of measuring the brightness for each block. That is, the method is such that those blocks other than a block to be measured (hereinafter also referred to as a measurement target block) are caused to turn off for a short period of time such as, for example, hundreds of microseconds, in order to remove the influence of ambient light, and then, the brightness of the measurement target block is measured during this period of time. However, it is found that this method has a problem to produce a flicker.
FIG. 13 is a view showing an example of the construction of a backlight and the order in which the measurement of brightness of each block is carried out by means of sensors, according to a conventional technology. In the example shown in FIG. 13, the backlight is composed of a total of 160 blocks including 16 pieces in the horizontal or transverse direction and 10 pieces in the vertical direction. In addition, acquisition of sensor values for each block is individually carried out on each of a left side surface (L) and a right side surface (R) of the backlight. FIG. 13 shows the order of acquiring the sensor values of the blocks on the left side surface for simplification of drawing. In the example shown in FIG. 13, with respect to the left side surface of the backlight, the acquisition of the sensor values is carried out in a sequential manner from an upper left block (sensor value acquisition start block) toward a lower end central block (sensor value acquisition end block). A set of 16 blocks in the transverse direction is called a “line”, and a set of 10 blocks in the vertical direction is called a “column”. In FIG. 13, line numbers 1 through 10 are given in order from the top of the backlight. In the left side surface, a block in the nth line from the top and in the mth column from the left side end is represented by a symbol L [m] [n]. Those blocks belonging to the same line are assumed to be driven and controlled by PWM signals at the same timing, respectively.
FIG. 14 is a timing chart which shows an example of PWM control of the backlight in the conventional technology shown in FIG. 13. In the example of FIG. 14, one frame period (60 Hz) is divided into five subframe periods (300 Hz), and the start timing of a turn-on period of time of PWM control is shifted line by line within one subframe period. According to this, turn-on control of the backlight is carried out in such a manner that those lines to be turned on within one subframe period move downward from the top to the bottom of the backlight (details will be described later by using FIG. 3 and FIG. 4). In general, PWM control is carried out for each line (i.e., line by line), but in cases where sensor values are acquired for each block (i.e., block by block), only a sensor value acquisition target block for which a sensor value is to be acquired is turned on, and those blocks other than the target block in a line to which the target block belongs are all turned off. In addition, those lines other than the line to which the sensor value acquisition target block belongs are also turned off.
FIG. 14 schematically shows an example of the timing chart which illustrates the PWM control of the backlight at the time of carrying out the acquisition of the sensor values of the blocks which belong to line 1. Here, it is assumed that a sensor value acquisition target block is caused to move for each subframe (i.e., subframe by subframe) in the order shown in FIG. 13. In a sensor value acquisition period in which a sensor value for a sensor value acquisition target block is acquired, those lines other than a line (e.g., line 1) to which the sensor value acquisition target block belongs are altogether turned off, including the lines (e.g., lines 8 through 10), too, which are originally to be turned on (i.e., in the turn-on period of time). In addition, even in the line 1, those blocks other than the sensor value acquisition target block are all turned off. For example, in cases where a block L [1] [1] at the left side end of the line 1 is the sensor value acquisition target block, the other blocks L [1] [n] (n=2 through 8) which belong to the line are turned off.
In FIG. 14, a portion indicating the PWM control of the line 1 is assumed to represent the PWM control of the sensor value acquisition target block in particular among the blocks belonging to the line 1. As mentioned above, the sensor value acquisition target block moves subframe by subframe, as shown in FIG. 13, so the PWM control described in the line 1 in FIG. 14 represents the PWM control of a different block for each subframe. For example, the portion of the first subframe represents the PWM control of a block L [1] [1] of the line 1. The PWM control of the other blocks L [1] [n] (n=2 through 8) in the line 1 is forced to turn off, similar to the lines through 10. The portion of the second subframe represents the PWM control of a block L [1] [2] of the line 1, and the PWM control of the other blocks L [1] [n] (n=1, 3 through 8) in the line 1 is forced to turn off. Thus, in a precise sense, in a line including a sensor value acquisition target block, PWM control is different for between the sensor value acquisition target block and the other blocks, but in FIG. 14, the description thereof is omitted in order to simplify the illustration.
FIG. 15 is a view schematically showing a temporal change in the turn-on states of the left side surface of the backlight in the case of carrying out the acquisition of sensor values of the blocks belonging to the line 1 of the backlight according to the timing chart shown in FIG. 14. Here, note that in FIG. 15, for the sake of simplified illustration, only a sensor value acquisition period and turn-on states immediately before and immediately after each sensor value acquisition period are extracted and described from among each subframe period. For example, in the first subframe period, there are described only a turn-on state at the time of carrying out sensor value acquisition for a block at the first column in the line 1 and turn-on states in which the lines 1, 8 through 10 are turned on immediately therebefore and immediately thereafter. After this, in actuality, a period of time in which the lines 1, 2, 9 and 10 are turned on, a period of time in which the lines 1, 2, 3 and 10 are turned on, and so on continue, but the description thereof is omitted.
In the first subframe period shown in FIG. 15, the lines 1, 8 through 10 are first turned on, and after the lapse of a predetermined period of time, a sensor value acquisition period in the line 1 comes, so that only a first sensor value acquisition target block L [1] [1] in the line 1 is turned on. In this sensor value acquisition period, the lines 8 through 10 and the blocks other than the block L [1] [1] in the line 1, which are originally in their turn-on periods of time, are all forced to turn off. At timing immediately after the end of this sensor value acquisition period, the lines 1, 8 through 10 are turned on again. In the second subframe period in FIG. 15, the lines 1, 8 through 10 are first turned on, and after the lapse of the predetermined period of time, a sensor value acquisition period in the line 1 comes, so that only the following sensor value acquisition target block L [1] [2] in the line 1 is turned on. In this sensor value acquisition period, the lines 8 through 10 and the blocks other than the block L [1] [2] in the line 1, which are originally in their turn-on periods of time, are all forced to turnoff. At timing immediately after the end of this sensor value acquisition period, the lines 1, 8 through 10 are turned on again.
During a period of time over a plurality of subframes in which measurements of the brightness of each block in the line 1 are carried out in this manner, forced turn-off periods of time are inserted in the turn-on periods of time of the lines 8 through 10 for each subframe, a large brightness variation will occur in a periodic manner. This may be recognized as a flicker.
In the examples of FIG. 14 and FIG. 15, in each sensor value acquisition period of the line 1, what is forced to turn off is the blocks in the three lines of the lines 8 through 10, but when a PWM control value (duty ratio) becomes a large value, the number of lines to be forcibly turned off in the sensor value acquisition periods will increase. For that reason, a brightness change between each sensor value acquisition period and each of the other periods will be larger, so it becomes easy to be recognized as a flicker.
However, human reaction to light is due to an amount of light which is obtained by integrating the light received by the eyes over a period of time of about 1/60 seconds. In addition, in cases where brightness changes rapidly between high intensity and low intensity, in particular in response to interruption of light, it is easy for human beings to recognize such a change as a flicker.
Japanese patent No. 4094952 describes a technique of reducing a flicker at the time of acquiring sensor values. In the technique described in this patent No. 4094952, in a white lighting apparatus using LEDs (Light Emitting Diodes) of R (red), G (green), and B (blue) as light sources, acquisition of sensor values is carried out during a measurement cycle of an LED of a certain color, by putting LEDs of the other two colors into off states. In this case, in the technique described in the above-mentioned patent No. 4094952, immediately before and immediately after the time when the LEDs of the other two colors are put into turn-off states, the intensities of the LEDs of the other two colors are caused to increase to a slight extent.